Memory devices are typically provided as internal, semiconductor, integrated circuits in computers or other electronic devices. There are many different types of memory including random-access memory (RAM), read only memory (ROM), dynamic random access memory (DRAM), synchronous dynamic random access memory (SDRAM), and flash memory.
Flash memory devices have developed into a popular source of non-volatile memory for a wide range of electronic applications. Flash memory devices typically use a one-transistor memory cell that allows for high memory densities, high reliability, and low power consumption. Common uses for flash memory include personal computers, personal digital assistants (PDAs), digital cameras, and cellular telephones. Program code and system data such as a basic input/output system (BIOS) are typically stored in flash memory devices for use in personal computer systems.
Flash memory cells are typically comprised of field effect transistors (FET) with floating gates. The gates are referred to as floating since they are electrically isolated from other conductive areas of the transistor by layers of oxide insulation. The floating gate can be programmed or erased by Fowler-Nordheim tunneling in which electrons tunnel through a barrier in the presence of a high electric field in the oxide.
One drawback with floating gate FETs is the relatively large amount of time needed to store a charge on the floating gate during a write operation and the relatively large amount of time necessary to remove the charge during an erase operation. One reason for the high time requirements is the relatively large tunneling barrier between the silicon substrate and the silicon dioxide insulator. Additionally, the high electric field required to cause electron injection in order to tunnel through the barrier typically contributes to reliability problems and premature gate insulator breakdowns.
As a typical prior art example, silicon dioxide (SiO2) is an insulator with a relative dielectric constant of 3.9, an energy gap of approximately Eg=9 eV, and electron affinity of χ=0.9 eV. By comparison, the energy gap and electron affinity for the semiconductor silicon are Eg=1.1 eV and χ=4.1 eV, respectively. In a conventional flash memory cell, electrons stored on the polysilicon floating gate see a large tunneling barrier of about 3.2 eV. FIG. 1 illustrates the typical prior art large barrier, Φ=3.2 eV, for tunneling erase in flash memory devices. The large tunneling barrier Φ=3.2 eV is the difference between the electron affinities of silicon (i.e., χ=4.1 eV) and SiO2 (i.e., χ=0.9 eV). This is a relatively large barrier that requires a high applied electric field.
There is a resulting need in the art for an improved gate insulator that provides a low tunneling barrier in order to decrease the time required for programming and erase operations in a flash memory cell.